(a) Field of the Invention
The present invention relates to an amplifier with power supply switching, more particularly to an audio power amplifier which is capable of effecting a signal amplifying operation with high power efficiency and with reduced switching noises and reduced distortions of signals.
(b) Description of the Prior Art
As a power amplifier for use in audio devices, there has been proposed and placed on the market an arrangement that a power supply voltage supplied from a power supply to the output stage of the power amplifier is varied in accordance with the level of either an input audio signal to be amplified, or of an amplified output audio signal, to thereby enhance the power efficiency of said output stage of the power amplifier.
For example, FIG. 1 illustrates a known such power amplifier, and shows a circuit diagram of the so-called E-class power amplifier. More particularly, the power amplifier shown in FIG. 1 is arranged so that a supply voltage V.sub.c powering an amplifying transistor 2 is varied in accordance with an audio signal level which is applied to an input terminal 1 of the power amplifier. Still more specifically, this known power amplifier is arranged so that, in case the input signal has a low signal level, the transistor 2 is operated by a voltage supplied from a power supply 3 via a diode 4, whereas in case the input audio signal has a high signal level, another transistor 6 which has been given base-biasing through a biasing source 5 is turned "on", so that the transistor 2 is operated by the sum voltage supplied from said power supply 3 and a power supply 7, to drive a load 8 by an operating current delivered by this transistor 2. According to this known power amplifier, it is possible to vary, as shown in FIG. 2, the supply voltage V.sub.c in accordance with the input audio signal level or with the output audio signal level V.sub.o. Thus, it is possible to reduce the collector loss of the transistor 2, to thereby enhance the power efficiency of the amplifier per se. Such known power amplifier has been disclosed in, for example, U.S. Pat. Nos. 4,115,742, 3,622,899 and Japanese Laying-open Patent Application No. 50-45549.
However, such known power amplifier as briefed above has the disadvantage that the transistor 2 becomes saturated at the moment at which the input signal level exceeds the voltage level of the power supply 3, and that, owing to the resistance of the saturated transistor 2, the amplifying degree G.sub.m of this transistor 2 drops, causing distortions to develop.
FIG. 3 illustrates another example of such known power amplifier as discussed above. The power amplifier shown in FIG. 3 is of the arrangement that an input signal applied to an input terminal 9 is voltage-amplified by a driver amplifying circuit 10, and is subjected to power-amplification by transistors 11 and 12 connected in complementary SEPP fashion, and features the provision of Schmitt trigger circuits 14 and 15 which operate so that one 14 of them detects the point of time at which the level of the output voltage supplied to a load 13 exceeds, with respect to the positive region of the voltage, a certain positive value, whereas the other Schmitt circuit 15 detects the point of time at which the output voltage level for the load 13 exceeds, with respect to the negative region of this voltage, a certain negative value, so that the resulting outputs of these Schmitt trigger circuits cause switching circuits 16 and 17 to perform on-off actions. Whereby, the power supply voltage applied to the transistor 11 is switched to a high level positive voltage +V.sub.H when the aforesaid output voltage in the positive region is above a certain value, and to a low level positive voltage +V.sub.L when said output voltage in the positive region is lower than the certain value. Likewise, the aforesaid voltage of the power supply is switched to a high level negative voltage -V.sub.H when the output voltage in the negative region is above a certain value, and to a low level negative voltage -V.sub.L when this output voltage in the negative region is lower than the certain value. Reference numerals 18 and 19 represent diodes for inhibiting reverse current flow. This known power amplifier is able to vary, as shown in the voltage waveshape diagram in FIG. 4, the level of the power supply voltage in accordance with the level of the output voltage, thereby enhancing the power efficiency of the power amplifier in much the same way as is done by the known power amplifier shown in FIG. 1. Such known power amplifier as shown in FIG. 3 has been disclosed in Japanese Laying-open Utility Model Application No. 49-59451 and U.S. Pat. No. 3,319,175.
It should be noted here, however, that the known power amplifier shown in FIG. 3 has the arrangement that the switching-over of the power supply voltage to different values can be performed for each one cycle of the output signal V.sub.o. Therefore, this power amplifier has the disadvantage that, in case the output signal level as well as its frequency are both high, the number of switching actions of the switching circuits 16 and 17 becomes great, so that the switching circuits 16 and 17 require switching elements capable of making high-speed switching actions, and that switching noises in the output signal become prominent accordingly.
As discussed above, those prior art power amplifiers shown in FIGS. 1 and 3 invariably provide a high power efficiency, but they have the operational disadvantages as stated above. From the nature of these disadvantages, however, such disadvantages cause a significant problem when used as a power amplifier for audio devices.